We wish to continue the approach of isolation and characterization of various mammalian cell mutants defective in specific lipid synthesis to study regulation of cholesterol metabolism. Our recent findings have led us to form the following working hypotheses: (1) rates of cellular cholesterol synthesis and unsaturated fatty acid synthesis may be coordinately regulated. (2) Activities of various enzymes involved in sterol synthesis may be coordinately regulated by a single regulatory signal. We propose various approaches to test our hypotheses and to explore the molecular mechanisms controlling such regulatory processes. Our planned experimentations are summarized as follows: Examine the detailed mechanism controlling coordinate regulation of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase and HMG-CoA reductase by low-density lipoprotein or by 25-hydroxycholesterol in wild-type Chinese hamster ovary (CHO) cells. Biochemical characterizations of four classes of CHO cell mutants defective in specific lipid synthesis. The first two classes of mutants seem to contain altered structural genes coding for specific enzymes, while the other two classes of mutants seem to be defective in regulation of sterol synthesis and unsaturated fatty acid synthesis. Purification of cytosolic HMG-CoA synthase from Syrian hamster liver and use of monospecific antibody against the homogeneous enzyme to study regulation of HMG-CoA synthase activity in CHO cells. Isolation of CHO cell mutant (s) with altered structural gene coding for HMG-CoA synthase.